1. Field of the Invention
This invention relates generally to an odor control system for wastewater settling tanks to capture and treat noxious and ordoriferous gases emitted therefrom, and more particularly to a system that includes a multi-section hood, each section of which is provided with an access port in which test probes are insertable to determine the velocity of the gas-laden air flowing under the hood and the concentration of the gases therein.
2. Status of Prior Art
Sedimentation is the process by which suspended particles heavier than water are removed by gravitational settling. Sedimentation which comes into play in the natural purification of streams and lakes is also widely used in the treatment of sewage and industrial wastes.
Systems for treating sewage and industrial wastes typically include preliminary settling tanks that collect a good part of the suspended of impurities before the clarified effluent is discharged into receiving waters or is subjected to further treatment in secondary settling tanks. In the secondary or final settling tanks, the clarified effluent from the primary tanks are further treated by additional settling and clarification to purify the effluent.
A settling tank has four functional zones; namely, an inlet zone, a settling zone, a sludge zone and an outlet zone. For high efficiency, inlets must distribute the incoming wastewater as uniformly as possible within the tank. In the sludge zone, sludge is worked into a sump from which it is withdrawn by gravity or by a pumping action. The main concern of the present invention is with the outlet zone in which the control of outflow is usually effected by a weir attached to one or both sides of an outlet trough of launder which acts as a lateral spillway.
The Anderson U.S. Pat. No. 4,391,704, discloses an outlet zone whose structure and operation is such as to prevent odoriferous and noxious fumes from entering the atmosphere. Because incoming waste is saturated with noxious gases and easily volatilized compounds, some degree of odor control is generally necessary in a liquid waste treatment system. In sewage, the objectionable compound which predominates is hydrogen sulfide, a colorless gas having an offensive odor. This gas is highly toxic and a strong irritant to eyes and mucous membranes. Hydrogen sulfide is almost always present in sewage, for it is the product of anaerobic decomposition by bacterial present in the sewage and in the sewage treatment system.
With settling tanks and sewage collection systems having significant detention times, the generation of hydrogen sulfide, particularly in warm weather, can be so vigorous that the gases released form the tank create serious problems. Apart form its unpleasant ordor, the presence of hydrogen sulfide in the atmosphere is hazardous to personnel in the vicinity of the tank and may cause permanent injury and even death. And since hydrogen sulfide will combine with moisture in the atmosphere and water on concrete or metal surfaces to form sulfuric acid, this acid may severely corrode the structures.
One known approach to odor control is to fully cover the settling tank. But this approach leaves much to be desired; for when the tank is covered with a concrete or other structure, the reaction of the gases with the structure then gives rise to serious corrosion problems. Moreover, the cost of such a cover is very high.
The outlet zone disclosed in the Anderson patent functions to localize the odoriferous and noxious fumes to the region of the collection trough or launder from which they are extracted, the settling zone of the tank being uncovered. The Anderson invention is based on the recognition that almost all of the noxious gases generated in a settling tank are released as the clarified water spills over the weirs mounted on the sides of the trough; hence by capturing these hazardous gases in the trough region and transporting the captured gases to a gas-control system or in otherwise disposing of the gases, one is able to reduce the discharge of gases into the atmosphere to an acceptably safe level.
The Anderson patent provides an outlet zone for a settling tank in which the trough is covered by a hood that also functions as a scum baffle. In a conventional outlet zone, a baffle which protrudes into the tank water is mounted adjacent to the weir of the trough to prevent grease and other floating matter from being discharged into the trough with the effluent. With the Anderson arrangement, the hood which acts to confine the gases released in the trough has a scum baffle integral therewith.
Inasmuch as the hood in Anderson which covers the trough is required to collect a relatively small volume of air as compared to a cover placed over the entire tank, a significant advantage of this arrangement is that it is not only more effective than a massive tank cover, but is also substantially less expensive to construct, operate and maintain.
Also of prior art interest is the Roley U.S. Pat. No. 4,767,536, which discloses an outlet for extracting noxious gases generated in a settling tank containing a pool of wastewater, the outlet including an effluent trough supported at a raised position in the pool against the tank wall. The trough is provided with a side wall parallel to the tank wall and lying somewhat below the surface of the pool. A weir, supported on this side wall and extending upwardly therefrom, permits clarified water from the surface region of the pool to spill over into the trough.
Overlying the trough and the weir is an odor control hood that includes a horizontal flat walk plate that extends from the tank to the upper edge of a vertical sidewall which extends downwardly into the pool. The sidewall together with the walk plate acts to confine noxious gases emitted into the region above the trough and the weir to prevent their discharge into the atmosphere. The walk plate includes a hatch cover section providing access to the weir when it becomes necessary to scrub and clean the weir.
In an existing settling tank odor control system of the type disclosed in the Roley patent, an external exhauster functions to draw an air stream having noxious gases entrained therein from the region underlying the hood and to feed this stream at a specified velocity to a scrubber. In this installation, the parameters are such as to change all of the gas-laden air beneath the hood several times per hour. The scrubber is designed to take this change rate into account and neutralize hydrogen sulfide (H.sub.2 S) and other highly odorous sewage gases such as indoles, skatoles, mercaptans and amines before the treated air is discharged into the atmosphere.
However, the rate at which noxious gases are generated and the volume of such gases captured by the hood depends on the nature of the wastewater being treated. Hence a given flow velocity setting may be inadequate in those situations where the concentration of noxious gases in the air stream becomes very high because of the changing character of the wastewater. Also, even if the nature of the wastewater does not change, the amount of noxious gases emitted therefrom varies with the seasons of the year. Thus in hot weather more gases are generated than in cool weather.
As noted previously, the presence of H.sub.2 S in the region beneath the hood can have a corrosive effect on the concrete walls defining the trough as well as on metal parts exposed to the gas, for H.sub.2 S combines with moisture in the air and with water on the concrete and metal surfaces to produce sulfuric acid. If, therefore, there are stagnant pockets of H.sub.2 S in the region under the hood or the flow velocity is too low to withdraw the gas from the hood confined region, over a period of time the resultant acid corrosion may be so damaging as to require major repairs or replacements.
It is essential, therefore, in order to calibrate an odor control system for a wastewater settling tank and maintain it in good working order, that the operator of the system be able periodically to check the gas-laden air stream velocity at a series of designated check points along the region under the hood covering the trough. From this measurement, the operator can determine whether the system is functioning properly under the prevailing conditions or whether correction is required because of some obstruction or impedance at some point in the gas-laden air stream path under the hood. It is also essential, in order to determine whether the system is functioning properly, to measure the concentration of gases at these points to be sure that there is no excessive rise in concentration.
In this way, adjustments can be made by the operator to maintain the odor control system in proper working order and thereby minimize the release of odoriferous gases into the atmosphere as well as to avoid corrosion. However, with an odor control hood arrangement of the types disclosed in the Anderson and Roley patents, the operator has no access to the trough region confined by the hood structure, and it is not possible to test the velocity of the gas-laden air stream flowing through the region or the concentration of gases therein.
Thus while the hood in the Roley patent is provided with hatch covers along the length of the trough, thereby giving access to the weir for cleaning purposes, when a hatch cover is raised the system is short-circuited and the airtight seal broken, thereby causing intereference with the specified air flow requirements.
One reason why it is important to know the prevailing gas concentration in the region under the hood is that in order to fully neutralize these gases, the scrubber into which the gases are fed must include chemicals for this purpose in an amount and strength sufficient for a predetermined volume and concentration of incoming gas. Hence in a given situation, a measurement of gas concentration may tell the operator of the system that the chemical charge in the scrubber is insufficient under prevailing conditions and must be increased.